The present invention relates to a recording and reproducing system for recording and reproducing digital information signal in a recording medium such as CD and an equalizing method for use in it. More particularly, it concerns a recording and reproducing system and an equalizing method for use in it that makes it possible to increase recording density of digital information signal without changing parameters inherent to a recording medium.
Conventionally, the art of such a technique has been disclosed in, for example, "Association of Television Technology", Vol. 42, No. 4 (April, 1988), pp. 330-337. The prior art is described below.
A digital information signal to be recorded in a recording medium, such as a CD, is modulated by a run length limited (hereinafter referred to as the RLL) coder with use of the so-called (d, k) RLL code of minimum run length d and maximum run length k before being recorded. The signal reproduced from the recording medium is equalized by an equalizer before being decoded to the original digital information signal by a decoder to feed out.
FIG. 9 shows a frequency response M(f) of the RLL coder and a general overall transmission frequency response H(f) of the RLL decoder, the recording medium, and the equalizer. Curves (a) and (b) in the figure indicate the frequency response M(f) and the overall transmission frequency response H(f), respectively. Fb, Fn, and Fm in the figure indicate a recording bit rate after the modulation, a Nyquist frequency (=(Fb/d)/2), and a cut-off frequency of the recording medium 3, respectively. In general, the frequency response E(f) of the equalizer is set so that as shown by curve (b), the overall transmission frequency response H(f) should become a raised cosine characteristic (100% roll-off factor) of a Nyquist frequency Fn. The recording bit rate Fb therefore can be increased until bandwidth B (=2.multidot.Fn) of the overall transmission frequency response H(f) become the cut-off frequency Fm. Hence, the recording bit rate Fb is given with respect to the cut-off frequency Fm as EQU Fb=d.multidot.Fm (1)
FIG. 10 shows an eye pattern of the output signal of the equalizer 4 with the minimum run length d being 3. If the recording medium 3 is an optical disk, the cut-off frequency Fm of the recording medium 3 is given by an equation below: ##EQU1##
where .lambda. denotes a wavelength of laser, NA is a numerical aperture of an optical system, and V is a linear velocity of the recording medium.
As an example, a CD system with the minimum run length d of 3 can be made to have the recording bit rate Fb of around 4.32 Mbps and the bandwidth B of around 1.44 MHz. From Eq. 2, the cut-off frequency Fm becomes around 1.44 MHz that is equal to the bandwidth B if the wavelength .lambda. of the laser is 0.78 .mu.m, the numerical aperture NA of the optical system is 0.45, and the linear velocity of the optical disk is 1.25 m/sec.